The infrared radiation having wavelengths greater than 2 .mu.m has been utilized in a variety of fields such as medical service, industrial working, measurement, analysis or chemistry and the like. In particular, Er-YAG laser having a 2.94 .mu.m wavelength band, CO laser of 5 .mu.m band, and CO.sub.2 laser of 10.6 .mu.m band have a high oscillation efficiency, a high output, and a high absorption coefficient of water and thus, they are noted as the radiation source of a laser treating device for medical or dental service.
Now, a silica optical fiber which is used for general communication is unsuitable to be used for a waveguide for long distance transmission, because the infrared absorption caused by molecular oscillation becomes large, when a laser light having a wavelength of more than 2 .mu.m is used therein. Accordingly, the transmission system composed of a material other than the silica optical fiber or various transmission systems having other designs have been proposed and have come in practice.
For example, in the Er-YAG laser for dental treatment being remarked at present, a fluoride glass fiber is used to guide laser light from the light source to the diseased part. The laser light having a wavelength of 2.94 .mu.m in the Er-YAG laser has the highest absorptance to water among the lasers oscillating in the infrared wavelength range. This laser does not be accompanied with heat or vibration as generated in cutting with an air turbine or electric motor, so that the patient does not feel a pain even under non-anesthesia. Therefore, this laser is capable of cutting the hard tissues like formation of cavities in treatment of carious teeth or removal of dental calculus.
In addition to the cutting of teeth, the medical treatment by the laser has a variety of effects such as incision of soft tissue, arrest of bleeding, sterilization, and the like and laser sources are used in accordance with the respective treating objects. The fluoride glass-fiber has a low loss at the wavelength range of the Er-YAG laser, but it has an increased loss at the wavelength range longer than that range, and thus a chalcogenide glass fiber has been examined for transmission of a CO laser beam. Further, the transmission through the glass fiber is difficult for a CO.sub.2 laser beam having further longer wavelength, so that a silver halide or thallium halide crystalline fiber is used therefor. In addition to the solid type of optical fibers, a hollow waveguide, particularly a metal hollow waveguide having a dielectric layer coated on the inside wall thereof and having the desired material and thickness for a particular wavelength of a laser beam to be transmitted has been proposed and examined.
The waveguide to transmit the laser light from the light source to the diseased part is housed within a complicated cable having the long tubes to pass water and gas and the like, and thus, it is technically and economically difficult to dispose the whole of this transmission system or to sterilize it for reuse for each medical treatment. Therefore, it is general means to connect a relatively short and exchangeable laser probe which is utilized for access to, contacting with or insertion into the diseased part, to the leading end of a long waveguide. Since this laser probe is directly exposed to the diseased part or the atmosphere of sterilization, it is required to have various characteristics such as low loss transmission, mechanical strength, heat resistance, resistance to water or chemicals, non-toxicity, easy-to-use, and the like. In order to reuse the laser probe at least, it must withstand the treatments such as sterilization by a high temperature vapor treatment or dipping into chemicals sterilization.
The laser probe is short, but it is one of waveguides to transmit a laser light, and thus, it is conceivable to employ the same material and structure as those of the long waveguide for transmitting the laser light from the light source to the diseased part to make the laser probe. However, the materials to be used for making the above mentioned solid type of infrared transmission optical fibers, that is, the fluoride glass, chalcogenide glass, silver halide or thallium halide crystals can not sufficiently satisfy the performances required for the laser probes described above.
That is, first, the waveguides composed of these materials have generally a low mechanical strength. Although it is capable of using the waveguide by inserting it into a tough metal pipe and the like, the deterioration in the characteristics may occur due to chemical factors in addition to the physical fracture caused by the external force. For example, the fluoride glass will be deliquescent upon exposure to the atmosphere of a high humidity and may break with a progress of crystallization. The silver halide crystals may not only be sensitized by short wavelength light resulting in an increased loss, but also may be chemically reacted upon contact with metals such as iron, copper, aluminium and the like, thereby resulting in degraded optical characteristics and mechanical strength. When the thallium crystal has been held for long periods in a bent state, it slips on the crystal plane resulting in the increase of ductile fracture and scattering loss. Further, these materials are chemically weak as well as mechanically and thus, they can not sufficiently withstand the treatments such as sterilization by a high temperature vapor treatment or dipping into chemicals.
Also, in order to use the laser probe for medical service, it is an essential condition that it is innocuous. The chalcogenide glass or thallium halide crystals contains toxic substances such as As, Se, Tl, etc. These materials have a high refractive index and a relatively low melting point, so that they are subject to the thermal fracture and transpiration in the emitting end of a laser light. Since not only broken slices or pieces but also transpirated vapor are toxic, when these substances invade into the human body, they seriously affect the human body.
From the reasons described above, the laser probes which are made of the materials such as fluoride glass, chalcogenide glass, silver halide or thallium halide crystal and the like, and which are directly contacted with or inserted into the human body can not be used for the medical lasers which have been developed hitherto.
On the other hand, the hollow waveguide has a harmless structure and a high mechanical strength, and is stable to the external atmosphere. However, the hollow waveguide has a problem that dust and/or moisture which deteriorate optical characteristics is liable to invade into the inside thereof. Therefore, it can not be used as such and thus, it is required to prevent the invasion of dust or moisture into the inside of the waveguide.
In the conventional Er-YAG laser for dental treatment described above, a dry air flow in the inside of the tube having a fluoride glass fiber therein is continued during transmitting a laser beam so as to prevent the degradation in mechanical strength of the glass fiber caused by moisture. This laser apparatus has the structure in such a way that it is intercepted from the external atmosphere so that the leading end is directly exposed to the exterior after irradiation by laser light. To the leading end of the glass fiber is connected a short silica glass fiber of 2-3 cm long as a laser probe through a ball lens. This silica glass laser probe is directly contacted with the diseased part and is exposed to the atmosphere such as sterilizing atmosphere and the like for reuse of the laser probe. The silica glass is much superior to the fluoride glass in reliability of the mechanical properties and heat resistance and can sufficiently withstand the inferior environment. However, the silica glass has a sharply increased loss at the wavelength range longer than 2 .mu.m, and thus, the laser light is attenuated to be a level of approximately 60% even through the laser probe having only a 2-3 cm long. This drop in transmission efficiency imposes new burdens such as improvements in a transmittance of a long waveguide or in an output capacity of a light source and the like on the laser apparatus. Further, there are cases where a laser probe having a length greater than 10 cm is required or the laser probe must be formed with a particular shape such as bent, curved shapes, etc. depending upon a particular object of medical treatment. The silica glass can not sufficiently meet these requirements in the respects of its loss or workability. Further, a short silica glass laser probe can barely be used for the wavelength of the Er-YAG laser, but even such a short silica laser probe can not be used for the CO laser or CO.sub.2 laser which oscillates at a longer wavelength
As described above, in the medical lasers which are now developing, the laser probes which are high in reliability, low in attenuation of a laser light, and safe and easy in use have hardly come in practice. They can not make full use of the inherent features of the laser treatment, and the effective cases applied to the laser treatment are very limited.